The coat protein of HIV-1, Gag, behaves on gels as if its molecular weight were 24kD, whereas the precursor protein is 55kD. The STEM is being used to examine proteins from an in vitro expression system to determine the mechanism of formation of capsomeres which are thought to be octamers. Several different cross-linking procedures have been tried to determine the intermediate stages in this multimerization. Approximately half of the cross-linked proteins were dimers and the rest had higher masses. Gag proteins were also cleaved into the constituent MA, CA, and NC domains. The MA and CA proteins were fixed and their masses analyzed by the STEM. The majority of the MA complexes were monomers. The CA complexes were heterogeneous, but their masses corresponded to tetramers, hexamers, and octamers suggesting they are oligomers of dimers.